Liquid crystal color screens are universally used in aircraft and helicopter cockpit viewing systems. They constitute an essential man-machine interface, providing the pilot, by means of elaborate symbolic images, with information which he requires in order to accomplish his various assignments. This displayed information must therefore be very reliable.
Now, it may happen that these screens exhibit display defects, and notably a defect termed frozen image, corresponding to a defect in the video display chain, generally due to an operating defect in the shift registers of the integrated line or column control circuits (drivers) by which the display of the video on the screen is supervised or to a lack of presence of the vertical scan synchronization signal at the input of the LCD screen.
The structure of a shift register is well known. Let us consider a shift register of n bits: this is a semi-conductor device comprising n stages in cascade, the output of each stage forming the input of the next. Each stage comprises a plurality of semi-conductor transistors. These transistors must ensure numerous switchings. Some of these transistors undergo a permanent gate stress, and this may give rise to a drift in their threshold voltage and hence a malfunction of the transistor: the transistor no longer switches. In a switching stage in which a transistor no longer switches, data transfer no longer takes place; the data output by this stage and by the following stages will therefore no longer change. As these are the shift registers of the line selection control circuit, the lines controlled by the output of these stages will therefore always remain in the same unselected state: scanning of the matrix selection lines no longer takes place. Let us assume that such an interruption of line scanning occurs. Having regard to the very high resistivity of liquid crystals and transistors in the off state, the pixels of an LCD screen have excellent information storage performance. The same image can thus remain displayed for several seconds, after this interruption.
Another display defect is the loss of video information in the image transmission chain, for example related to a failure of a color video pathway. For example, the color red is used to display alert signals. It is conceivable that a failure of the red video pathway may not be detected swiftly by the pilot on an operational image. In this case, the pilot's reaction may be too sluggish. It is thus necessary to be able to identify this defect.
A pilot may not notice a display defect, all the more when certain symbolic images, associated with information useful to the pilot, do not vary very quickly. He might therefore continue to trust the displayed image, although it is incorrect or no longer correct. Civil avionics safety recommendations prohibit this type of event. It is therefore necessary to provide a system for detecting a display defect.
According to the state of the art, for the selection line addressing circuit, the detection of this defect is usually carried out by making certain, in the output signal of the last stage of the shift registers, of the synchronous presence of the line scan signal on this output.
This scheme has various drawbacks. It makes it necessary to be able to physically measure the signal at the output of the last stage, and therefore to provide an additional conductor line, dedicated to this measurement. Furthermore, the information measured is that of the last line of the shift register. But the defect may lie further on, at the level of the voltage boosting circuit which is usually provided between the outputs of the shift register and the rows of the matrix, so as to pass from the digital voltage levels, in the shift registers, to the analog voltage levels necessary for controlling the image dots.
For the circuit for controlling the display of the video data on the columns, the defect detection consists in detecting the presence of a video signal at the input of the column control circuits, this being very insufficient. Notably this does not give any information about the operation of the shift register and/or of the digital analog conversion circuit and/or of the amplifying circuit for the column control devices and does not make it possible to be certain of the integrity of the display of one color in particular. Now, in the civil avionics context, the color red corresponds to the display of safety-related information. There is therefore a definite benefit in being able to be certain of the integrity of the display chain for this color at least.